1. Field of the Invention
The present invention relates to a method for a continuous production of magnetic metal particles and an apparatus therefor. More particularly, it relates to a method and apparatus for a continuous production of magnetic metal particles which are useful for magnetic recordings.
2. Discussion of the Related Art
Recent development of various recording systems has been remarkable, and among them, the advance of the reduction in size and weight of a magnetic recording/reproducing apparatus is significant. With this advance, higher performance on the magnetic recording media such as magnetic tapes and magnetic disks has been increasingly in demand.
In order to meet such demands on the magnetic recordings, magnetic particles having a high coercivity and a high saturation magnetization (us) are required. Conventionally, as the magnetic particles for magnetic recordings, acicular magnetite and maghemite or the so-called "cobalt-containing iron oxide" prepared by modifying these magnetic iron oxide particles with cobalt have been used. For the purpose of producing a higher output magnetic medium, ferromagnetic metal particles having a higher coercivity and a saturation magnetization, i.e., the so-called "magnetic metal particles," have begun to be used.
Such magnetic metal particles are usually produced by thermally reducing iron compound particles based on acicular iron oxyhydroxide or iron oxide to metallic iron in a reducing gas atmosphere such as a hydrogen gas stream. In this method, the acicular particles used as the starting material are reduced to form the so-called "skeleton particles," while retaining their original shapes. These skeleton particles comprise fine small unit particles, namely, crystallites, the small unit particles being connected to each other to form a skeletal structure. As this reduction reaction is carried out at a high temperature, particle crystallinity improves, and the saturation magnetization of the magnetic metal particles increases. However, such a high temperature reduction results in collapse of the acicular shape of skeleton particles and mutual sintering of skeleton particles, posing a problem of deterioration of the magnetic properties such as a coercivity and a squareness ratio (.sigma.r/.sigma.s) of the magnetic metal particles. To obtain magnetic metal particles of satisfactory performance, it is, therefore, necessary to solve the problem concerning how to retain the acicular shape of raw material particles in producing the desired magnetic metal particles.
Traditionally, various methods for reduction have been proposed to solve this problem, including 2) the method wherein reduction is conducted using a fluidized bed reduction furnace after granulation to pellets of 6 to 250 mesh size (Japanese Patent Laid-Open No. 174509/1983), 2) the method wherein reduction is conducted using a reactor equipped with an impeller blade (Japanese Patent Laid-Open No. 157214/1980), 3) the method wherein hydrogen reduction is conducted using a fixed bed reactor (Japanese Patent Examined Publication No. 48563/1985), 4) the method wherein hydrogen reduction is conducted using a cylindrical reduction furnace after granulation to 0.5 to 30 mm pellets (Japanese Patent Examined Publication No. 52442/1989, U.S. Pat. No. 4,400,337), and 5) the method wherein reduction is conducted using a rotary kiln after granulation to spherical pellets of 1 to 10 mm size (Japanese Patent Laid-Open No. 197506/1984).
Of these methods, reduction methods 1) and 2) have drawbacks of magnetic property deterioration due to promoted aggregation of the skeleton particles as a result of mutual contact or collision of the pellets, and dust escape from the reactor.
The fixed bed reduction methods 3) and 4) offer solutions to the above problems, but these methods have the following drawbacks: The hydrogen reduction reaction of iron oxide proceeds in two stages represented by the following formulas: EQU 3Fe.sub.2 O.sub.3 +H.sub.2 .fwdarw.2Fe.sub.3 O.sub.4 +H.sub.2 O (1) EQU Fe.sub.3 O.sub.4 +4H.sub.2 .fwdarw.3Fe+4H.sub.2 O (2)
In the fixed bed, because steam formed upon this reaction is accumulated as the raw material particle layer height (layer thickness) increases, the upper portion of the layer has a higher steam partial pressure. This steam promotes growth of the crystallites constituting the acicular skeleton particles. When the size of the crystallite is too large, the deformation of acicular shape and mutual sintering of the skeleton particles take place. Thus, as the layer height increases, the magnetic properties of the obtained particles are deteriorated. In addition, because reaction (2) is reversible, as the layer height increases, the reduction reaction rate decreases under more influence of the steam formed, resulting in uneven reduction. Although uniform magnetic metal particles of excellent magnetic properties can be obtained by lowering the ratio of the layer height to the tower diameter of the fixed bed, such fixed bed batch-wise reduction is unsuitable for industrial application because of very poor productivity.
In reduction method 5), contact between a reducing gas and a material to be reduced is insufficient because the reducing gas flows above the material layer, so that reduction time is longer than in methods 1) through 3). This poses a problem of tendency toward morphological change in acicular skeleton particles and mutual sintering of the particles.
There is, therefore, a need for the development of a method and apparatus for a continuous mass-production, on an industrial scale at a high efficiency, of the magnetic metal particles of excellent magnetic properties by preventing morphological changes in particles and mutual sintering of the particles in producing such fine magnetic metal particles.
Also, the magnetic metal particles obtained via such a reduction process are chemically unstable, undergoing oxidation in air, thus having a drawback of magnetic property deterioration with time. To overcome this drawback, various attempts, with proposals of various methods, have been made to stabilize the magnetic metal particles obtained by a thermal reduction as described above by further forming an oxidized layer on the surface thereof.
A conventional method for stabilization of the magnetic metal particles is the so-called liquid phase oxidation (e.g., Japanese Patent Laid-Open No. 128202/1985), in which the magnetic metal particles to be stabilized are suspended in a solvent, and an oxidizing gas is sparged into the suspension. However, this method has drawbacks such as adverse effect of oxidized solvent on coating and solvent handling safety assurance. Another method is the so-called gas-phase oxidation (e.g., Japanese Patent Examined Publication No. 14081/1984), in which an oxidized layer is formed by using a gas of adjusted oxygen partial pressure in a gas phase. At present, this gas-phase oxidation method is common.
In such gas-phase oxidation, a fluidized bed, which offers good contact between gas and solid is often used (e.g., Japanese Patent Examined Publication No. 14081/1984 and Japanese Patent Laid-Open Nos. 110701/1984 and 192103/1990). However, the gas-phase oxidation method using a fluidized bed has the drawbacks of magnetic property deterioration due to promoted aggregation of particles as a result of mutual contact or collision of pellets, and dust escape from the reactor.
On the other hand, if a gas-phase oxidation is possible in a stationary state of the magnetic metal particles to be stabilized, i.e., in a fixed bed, the above drawbacks can be overcome. However, this method for stabilization has the following drawbacks: The saturation magnetization (.sigma.s) of the magnetic metal particles decreases upon gas-phase oxidation, the degree of this decrease depending solely on gas-phase oxidation temperature. When a gas-phase oxidation is conducted by using a fixed bed, the heat of reaction generated by the oxidation reaction accumulates locally to heat only the portion to a high temperature, resulting in excessive decrease of saturation magnetization, and a non-oxidized portion is formed due to gas flow channelling. As a result, the obtained magnetic metal particles display a very wide fluctuation in the saturation magnetization. In some cases, upon exposure to the atmosphere, the non-oxidized portion becomes hot or ignites by a rapid oxidation reaction, which may significantly impair the essential coercivity and saturation magnetization. Moreover, such local accumulation of the heat of reaction and gas flow channellings are more likely to take place as the layer height on the fixed bed increases. For this reason, the magnetic metal particles having a uniform oxidized layer can be obtained by lowering the ratio of the layer height to the tower diameter of the fixed bed. However, such a fixed bed batch-wise gas-phase oxidation is unsuitable for industrial application because of very poor productivity.
Accordingly, there is a need for the development of a method and apparatus for a continuous mass-production, on an industrial scale at a high efficiency, of magnetic metal particles which have a uniform oxidized layer and are free from deterioration of the magnetic properties and unevenness in the properties, particularly in the saturation magnetization, upon stabilization by a gas-phase oxidation.